1. Field of the Invention
The present invention relates to an optical pick-up head for use in an optical reading and/or recording apparatus for reading and/or recording information from and/or on an optical information record medium, particularly a magneto-optical record medium. The present invention also relates to an integrated type optical unit for use in the above mentioned optical pick-up head.
2. Related Art Statement
FIG. 1 is a schematic view showing a first known optical pick-up head. The optical pick-up head reads and/or records information from and/or on a magneto-optical record medium. A linearly polarized laser beam emitted by a semiconductor laser such as a laser diode 1 is made incident upon a polarizing beam splitter 2 and a laser beam reflected by a polarizing film 2a provided in a contact surface between prisms is projected by an objective lens 3 onto an information track 4a of an magneto-optical information record medium 4 as a fine spot. The polarizing film 2a of the polarizing beam splitter 2 has a transmissivity of 60-90% for a component vibrating in a direction perpendicular to a plane of the drawing of FIG. 1 (S-polarized beam) and a transmissivity of substantially 100% for a component vibrating in a direction parallel with the plane of the drawing (P-polarized beam). Such a polarizing film 2a may be formed by a multiple coatings of dielectric material films. The linearly polarized laser beam emitted by the semiconductor laser 1 is made incident upon the polarizing film 2a as S-polarized beam.
The laser beam is reflected by the magneto-optical record medium 4 is subjected to the Kerr rotation and its polarizing direction is rotated about an optical axis by .+-..theta..sub.k depending upon information recorded on the record medium. The thus reflected return beam is converged by the objective lens 3 and is made incident again upon the polarizing beam splitter 2 as the converging light beam. Then, the return beam is transmitted through the polarizing film 2a so that the return beam is separated from the incident laser beam. Then, the return beam is made incident upon a multi-image prism 5. The multi-image prism 5 is consisting of a first triangular prism 5a and a second triangular prism 5, each being made of birefringent material. In order to detect an information signal recorded on the magneto-optical record medium 4 by a so-called differential method, an optic axis of the first prism 5a is set to be perpendicular to an optical axis of the return beam and to be inclined by 45.degree. with respect to a direction perpendicular to the plane of the drawing, and an optic axis of the second prism 5b is set to be inclined by 45.degree. with respect to the optic axis of the first primes in, for instance in the direction perpendicular to the plane of the drawing. Therefore, the return beam impinging upon the multi-image prism 5 is divided into substantially three light fluxes. The information signal read out of the optical record medium is denoted as MO signal for sake of simplicity.
The three light fluxes emanating from the multi-image prism 5 are then made incident upon a signal detecting photodetector 6 via a toric lens 7. The toric lens 7 has a function for extending a focal length of the transmitted light as well as a function as a cylindrical lens for introducing astigmatism which is required to detect a focusing error signal (hereinafter the focusing error signal is called FES). As illustrated in FIG. 2, the signal detecting photodetector 6 comprises three light receiving units 6a, 6b and 6c, and the middle light receiving unit 6b includes four light receiving regions. The MO signal is derived from a comparator 8 producing a difference between outputs of the light receiving units 6a and 6c, and the FES is obtained from a comparator 9 producing a difference between a sum of outputs of diagonally aligned light receiving regions of the light receiving unit 6b and a sum of outputs of other diagonally aligned light receiving regions. A tracking error signal (denoted by TES) may be derived by, for instance a push-pull (PP) method.
In Japanese Patent Application Laid-open Publication Kokai Hei 5-314563, there is disclosed a second known optical pick-up head. In this optical pick-up head, in a converged return beam there is arranged a plane parallel plate having an optical anisotropy, said plane parallel plate having a function of separating the return beam from the incident beam, a function of performing the polarizing separation for detecting the MO signal by the differential method, and a function of introducing astigmatism for detecting the FES. Ordinary light and extraordinary light separated by the plane parallel plate are received separately by a signal detecting photodetector.
The above mentioned first known optical pick-up head has the following problems:
(1) The polarizing beam splitter 2 for separating the return light path from the incident light path, the multi-image prism 5 for dividing the return beam into a plurality of light fluxes with the aid of the polarization, and the toric lens 7 for introducing the astigmatism required for detecting the FES are arranged separately from each other, and thus the optical pick-up head could not be made light in weight, small in size and cheap in cost. PA1 (2) The semiconductor laser 1 and signal detecting photodetector 6 which are arranged in conjugate with one another are spatially separated from each other by a large distance, and thus the optical pick-up head is liable to be subjected to temperature variation, secular variation, and others, so that use of the optical head is limited by ambient conditions. In order to mitigate such a drawback, it is required to use a large and expensive housing incorporating the optical elements. PA1 (3) An optical path length of the detecting optical system is increased by a concave lens function of the toric lens 7, so that size of the optical head is prolonged, although adjustment of the signal detecting photodetector 6 becomes easy. PA1 (4) A direction in which the astigmatism for detecting the FES is introduced is in parallel with a track direction in which an information track extends or is perpendicular to the track direction. Therefore, a push-pull signal for detecting the tracking error is leaked into the FES and the focusing servo is liable to be unstable. PA1 (5) The FES is detected only from one of the polarizing split beams, and thus an amplitude of the FES fluctuates due to a birefingency of a substrate of the magneto-optical record medium and an accurate focus servo control could not be performed. PA1 (6) A positioning of the signal detecting photodetector is difficult. PA1 (7) In the first known optical pick-up head shown in FIGS. 1 and 2, the number of optical elements is large, so that a total error represented by a sum of errors of respective optical elements becomes very large. In order to absorb these errors, the photodetector 6 has to be adjusted in xyz axes or the photodetector has to be adjusted in xy axes and the toric lens 7 has to be adjusted in z axis. In this manner, it is required to perform a very cumbersome three axis adjustment. In this case, the xy axis adjustment could not be independent from the z axis adjustment, so that a number of adjusting steps are increased. PA1 (8) Moreover, since the number of optical elements provided within a portion 10 surrounded by a chain line in FIG. 1 is large, an actual size of the portion 10 amounts to several tens to several hundreds millimeters by considering a space for providing adjusting mechanisms. Therefore, it is required an expensive housing consisting of a several blocks and complicated workings are required in order to manufacture these blocks. PA1 a semiconductor laser emitting a linearly polarized light flux; PA1 an objective lens projecting said light flux onto a magneto-optical record medium as a fine spot; PA1 a multi-image plane parallel plate arranged between said semiconductor laser and the objective lens in a converged return light flux reflected by said record medium, reflecting the linearly polarized light flux emitted by said semiconductor laser toward said objective lens and transmitting and refracting said return light flux to introduce astigmatism in the return light flux and to perform a polarizing beam splitting, said multi-image plane parallel plate including a first triangular prism and a second triangular prism which are made of birefringent material and are joined with each other; and PA1 a signal detecting photodetector receiving a plurality of light fluxes emanating from said multi-image plane parallel plate, detecting an information signal from outputs corresponding to mutually orthogonally polarized light components, and detecting a focusing error signal from outputs corresponding to ordinary and extraordinary light components; wherein said multi-image plane parallel plate is arranged such that directions of major and minor axes of said astigmatism are inclined by 45 degrees with respect to a track direction in which an information track on the magneto-optical record medium extends, an optic axis of said first triangular prism is set such that said return light flux is divided into the ordinary light and extraordinary light having substantially identical intensities, a polarizing film is provided on a surface of the first triangular prism upon which said linearly polarized light flux and return light beam are made incident, and an optic axis of said second triangular prism is set to be inclined by a predetermined angle with respect to the optic axis of the first triangular prism. PA1 a mounting substrate including a silicon wafer having a (110) uppermost surface and having formed therein upright recesses or upright walls each formed by {111} surfaces; PA1 a semiconductor laser secured to one or more upright recesses or upright walls, positioned at least in a direction of an optical axis of the semiconductor laser and emitting a light beam; PA1 a photodetector secured to one or more upright recesses or upright walls, positioned at least in a direction of an optical axis of the photodetector and receiving a return beam reflected by an optical record medium; and PA1 an optical path separating element secured to one or more upright recesses or upright walls, positioned at least in a direction of an optical axis thereof, directing the light beam emitted by the semiconductor laser toward the optical record medium and directing the return beam toward the photodetector. PA1 a mounting substrate including a silicon wafer having a (100) uppermost surface and having formed therein a plurality of pyramid recesses with {111} side walls; PA1 a semiconductor laser secured to a pyramid recess of the mounting substrate, positioned at least in a direction of an optical axis of the semiconductor laser and emitting a light beam; PA1 a photodetector secured to a pyramid recess, positioned at least in a direction of an optical axis of the photodetector and receiving a return beam reflected by an optical record medium; and PA1 an optical path separating element secured to a pyramid recess, positioned at least in a direction of an optical axis thereof, directing the light beam emitted by the semiconductor laser toward the optical record medium and directing the return beam toward the photodetector. PA1 a mounting substrate including an uppermost polyimide film having a plurality of upright recesses; PA1 a semiconductor laser secured to one of said upright recesses of the mounting substrate, positioned at least in a direction of an optical axis of the semiconductor laser and emitting a light beam; PA1 a photodetector secured to another one of said upright recesses of the mounting substrate, positioned at least in a direction of an optical axis of the photodetector and receiving a return beam reflected by the optical record medium; and PA1 an optical path separating element secured to another one of said upright recesses of the mounting substrate, positioned at least in a direction of an optical axis of the optical path separating element, directing the light beam emitted by the semiconductor laser toward the optical record medium and directing the return beam reflected by the optical record medium toward the photodetector. PA1 a mounting substrate including a silicon wafer having a (110) uppermost surface and having formed therein a plurality of upright walls formed by {111} surfaces; PA1 a semiconductor laser secured to one or more (111) upright walls and emitting a light beam; PA1 a photodetector secured to one or more (111) upright walls different from said one or more upright walls to which said semiconductor laser is secured, and receiving a return beam reflected by the optical record medium; and PA1 an optical path separating element secured to at least two convex corners of upright walls different from the upright walls to which said semiconductor laser and photodetector are secured, a rotation of said optical path separating element being inhibited by said at least two upright walls, directing the light beam emitted by the semiconductor laser toward the optical record medium and directing the return beam reflected by the optical record medium toward the photodetector.
In the second known optical pick-up head, the plane parallel plate having optical anisotropy has all the three functions, i.e. separating the return beam from the incident beam, polarizing beam splitting function and astigmatism introducing function, the above mentioned problems (2) and (3) of the first known optical pick-up head can be mitigated. However, the second known optical pick-up head has the following problems:
In known optical pick-up heads, a housing for accommodating the optical elements is manufactured by a mechanical process by using lathe, miller and NC lathe, and then the optical elements are fixed to the housing by means of screws and adhesive agent. Therefore, the housing is liable to become large and moreover positioning errors of the optical elements could be hardly limited to micron order.
In Japanese Patent Application Laid-open Publications Kokai Sho 62-197931 and 62-283430, there is disclosed a third known optical pick-up head. In this optical pick-up head, on a semiconductor substrate having photo diodes formed therein there are provided a beam splitter and a laser diode to constitute a single integrated body. This optical pick-up head can reduce a cost and a size. Moreover, in Japanese Patent Application Laid-open Publication Kokai Hei 3-44086, there is proposed a laser unit for use in the optical pick-up head. In this laser unit, a laser diode is sealed within a housing in which a collimator lens is provided.
If the above mentioned third known optical pickup head is applied to an infinite optical system using the collimator lens, it is necessary to adjust positions of a substrate on which the beam splitter and laser diode are secured with respect to an optical axis of the collimator lens, and this requires an adjusting mechanism and fixing parts therefor as well as spaces for the these elements. In this manner, although the beam splitter and laser diode are secured to the substrate in which the photo diodes are formed, size and cost of the optical pick-up head could not be reduced sufficiently.
In the laser unit described in the above mentioned Japanese Patent Application Laid-open Publication Kokai Hei 3-44086, the collimator lens is secured to the housing in such a manner that an outer periphery of the collimator lens is cemented to an inner wall of a circular opening formed in the housing. Therefore, the collimator lens could not be moved in a direction perpendicular to the optical axis of the laser diode, so that an adjustment of a propagating direction of a collimated beam is limited. Further, in this laser unit, a distance between the laser diode and the collimator lens could not be determined accurately, so that an accurately collimated laser beam has to be obtained by changing an oscillation frequency of the laser diode by adjusting an injection current therefor. Therefore, the laser unit could not be applied to an optical pick-up head in which a power of the laser beam is changed between the reading and the recording by adjusting the injection current to the laser diode. Moreover, the semiconductor laser whose wavelength is varied in accordance with the injection current could not be manufactured at a large scale at present and is expensive.